High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly advantageous as film and fiber-formers.
For certain application, such as injection or extrusion blow molding or blown film extrusion, it is desirable to use polyester resins of increased melt elasticity and/or melt viscosity. This is particularly desirable with poly(1,4-butylene terephthalate) which has a relatively low melt elasticity, even at very high molecular weights. Increases in melt elasticity and melt viscosity can be induced in a somewhat conventional way by including ester forming ingredients with a functionality of greater than two, such as glycerol or trimellitic esters, in the polymerization recipe. However, such expedients result in the formation of highly viscous melts, which are very difficult to remove from commonly used polymerization equipment. Moreover, during removal, such products are subject to thermal degradation, and there are economic losses due to lower production rates.
It has now been discovered that polyesters can be converted to compositions having increased melt elasticity and/or melt viscosity, rapidly and economically, by blending them with small amounts of organic compounds containing oxirane type oxygen, specifically organic compounds which are polyepoxides, and then heating the blend, e.g., in an extruder, to melt the mixture. This process causes some type of interaction, the nature of which is not clearly understood, but the final result is a composition uniquely adaptable to blow molding and blown film extrusion. Surprisingly, even though a polyfunctional epoxide is used, there is apparently no decrease in the thermoplastic nature of the linear polyester component.
Merely by way of illustration, poly(1,4-butylene terephthalate) having a melt viscosity of 6950 poises (at 250.degree. C.) and a nominal die swell (a measure of melt elasticity) on extrusion of 30%, when dried and blended with 0.5 and 1.0%, respectively, of bis( 3,4-epoxycyclohexylmethyl) adipate and extruded at 450.degree.-550.degree. F., has the melt viscosity increased to 9400 and 17,900 poises and the die swelling increased to 41% and 56%, respectively.